Filtration membranes and method of making the same

ABSTRACT

Microporous membranes can be prepared which comprise a polymer which is a hydrophobic polymer in bulk form and which has an equilibrium water absorption from about 2% to about 4%, but is hydrophilic when precipitated as a membrane. The membranes are further characterized by high water flow rates at any given bubble point.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.052,699, filed May 20, 1987, now U.S. Pat. No. 4,900,449, which is acomposite of copending U.S. patent application Ser. No. 050,052, filedMay 13, 1987, now abandoned, and copending U.S. patent application Ser.No. 011,461, filed Feb. 2, 1987, now abandoned. U.S. patent applicationSer. No. 050,052 is a continuation of U.S. patent application Ser. No.897,045, filed Aug. 15, 1986, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 812,260 filedDec. 23, 1985, now abandoned. U.S. patent application Ser. No. 011,461is a continuation of U.S. patent application Ser. No. 812,343 filed Dec.23, 1985, now abandoned.

TECHNICAL FIELD

The instant invention relates to filtration membranes. These membranescan be used for the production of particle and bacteria free water orsolutions. Such membranes are particularly useful in the electronic andpharmaceutical industries.

BACKGROUND ART

In aqueous microfiltration, such as the production of particle freewater or solutions in the electronics and pharmaceutical industries, itis highly desirable to have membranes with as little leachable materialas possible. Also, it is usually desired that the membranes be easilywettable by water or aqueous solutions. Therefore, membranes that areinherently water wettable (i.e., inherently water wettable as preformedmembranes without having to be post treated with a wetting agent) areoften preferred over hydrophobic membranes post treated by a wettingagent. That is, it is common to manufacture hydrophilic membranes byadding a wetting agent, such as a surfactant, to preformed hydrophobicmembranes. Upon use of the treated hydrophobic membranes, however, it isalways possible that the wetting agent may be leached by the solutionbeing filtered. Such leaching, in turn, may result in contamination ofthe filtrate.

At the present time, there are very few inherently wettable membranesthat have been identified. Generally, these membranes are made of nylon.Examples of such membranes are disclosed in the U.S. Pat. No. 3,876,738to Marinoccio et al, issued Apr. 8, 1975, and 4,340,479 to Pall, issuedJuly 20, 1982.

Polymers which are inherently very hydrophilic are difficult tomanufacture or have other drawbacks. Thus cellulose acetate membranesare limited in their heat and hydrolytic stability. Nylon membranes haveto be made with solvent systems that are hazardous and hard to handle.Polymers which are even more hydrophilic such as polyvinyl alcohol,polyvinylpyrrolidone and the like are difficult or impossible tocoagulate under practically feasible conditions because of their highaffinity to water. Therefore, it is not generally convenient to makehydrophilic membranes out of highly hydrophilic polymers.

An ideal bulk polymer for a microfiltration membrane formation would beone which is inherently hydrophobic, to facilitate membrane formation,and would also impart desirable mechanical and thermal properties.However, the membrane made from such a polymer would also need to beinherently or easily water wettable by suitable means, e.g., by adding awetting agent as noted above, such as a surfactant to facilitate aqueousfiltration, but doing so in such a way as not to involve later aqueousleaching of the wetting agent and thus causing contamination of thefiltrate. This invention relates to a class of polymers surprisinglyfulfilling these, apparently conflicting, requirements.

Polyamide, polyimide, polysulfone and polyethersulfone polymers havebeen used in the preparation of membranes. For example, U.S. Pat. No.3,816,303 to Wrasidlo, issued June 11, 1974, discloses a process fordesalination of saline water by reverse osmosis using a membranecomprising a film of poly(N-amido)imides having a specific formulation.The Wrasidlo patent describes a reverse osmosis membrane made fromspecific polyimide structures. The reverse osmosis membranes have aporosity significantly different than those found in microporousmembranes. Fully aromatic polyamides have been used to make reverseosmosis membranes as exemplified by U.S. Pat. No. 3,172,741 to Jolley.Polyimides have been developed for gas separating membranes, asdisclosed in Japanese document JP 58 08,514.

Polyimide polymers are usually insoluble and need to be formed in situ(that is, the membranes need to be synthesized) in the polyamic acidform and then heat treated to form the final polyimide membrane.Aromatic polyamides are not easily available as a pure bulk polymer andtherefore usually need to be synthesized by a membrane producer. Unlikethe aforementioned polymers, polyamideimide is commercially available atreasonable cost and is soluble in commonly used solvents.

The U.S. Pat. No. 3,719,640 to Lester et al, issued Mar. 6, 1973,discloses linear polymers of polyamideimides having a specificformulation containing a quaternizable nitrogen atom. The quaternizingof the nitrogen is pH dependent. When the nitrogen is quaternized, thepolymer is hygroscopic and may be used as separatory membranes in suchprocesses as desalination.

The U.S. Pat. Nos. 3,855,122 to Bourganel, issued Dec, 17, 1974, and4,286,015 to Yoshida et al, issued Sept. 28, 1982 disclose membranesmade from polyaryl ethersulphones having specific structuralformulations.

The Bourganel patent discloses an asymmetric membrane having a porosityin the reverse osmosis/ultrafiltration range. The Bourganel patent onlydiscloses membranes made from sulfonated polysulfone. The Yoshida et alpatent also discloses asymmetric ultrafiltration membranes made frompolysulfone.

The U.S. Pat. No. 4,240,914 to Iwama et al, issued Dec. 23, 1980,discloses a membrane and process for making the same made from analiphatic polyimide polymer.

None of the aforementioned patents disclose an intrinsically hydrophilicmembrane having a porosity in the microporous range.

The Japanese patent document to Shou 54-26283 describes a method ofmaking apparently microporous membranes from polysulfone polymers withhigh molecular weight polyethyleneglycol as an additive. Commonmicrofiltration membrane characterization tests (such as bubble points,bacteria retention) are not reported and no distinction is made betweenthe various possible polysulfones.

The present invention provides a membrane made of a hydrophobic polymerwhich, independent of pH, is surprisingly water wettable in amicroporous membrane structure.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided amicroporous membrane which comprises a polymer selected from a group ofpolymers which are characterized in that they are: (1) inherentlyhydrophobic in bulk form, and (2) hydrophilic as the polymerizedmembrane or hydrophilic as the polymerized membrane when containing 1 to6 percent by weight polyethyleneglycol. The instant invention furtherprovides a method of making the microporous membrane comprising thesteps of dissolving the aforementioned polymer in a solvent to form apolymer solution, forming a thin film of the polymer solution,precipitating the polymer as a microporous membrane, and drying themicroporous membrane.

DETAILED DESCRIPTION OF THE INVENTION

The solubility parameter of polymers measures the cohesive energy perunit volume and is generally associated with polarity. The greater thepolar character of the polymer's functional groups, the higher thesolubility parameter. For this reason, it is the usual observation thatpolymers with higher solubility parameters are more wettable (have lowercontact angles with water, a highly polar liquid). However, this orderseems to be reversed in some of the subject polymers characterizing theinstant invention. For example, membranes made of polyethersulfone, witha solubility parameter of 20.6 MJ/m³, are more wettable than polysulfonemembranes with a solubility parameter of 21.7 MJ/m³, as disclosed in theU.S. Pat. No. 4,387,187 to Newton issued June 7, 1983.

The present invention is the first instance in microfiltration membraneapplications in which a clear advantage of one of these polymers overthe other has been identified. It is another surprising aspect of thisinvention that this advantage is revealed only in the microporous rangeof porosity.

The membrane according to the instant invention has a microporousporosity and comprises a polymerized polymer selected from a group ofpolymers which are characterized in that they are: (1) inherentlyhydrophobic in bulk form, and (2) hydrophilic when precipitated as amembrane or hydrophilic when precipitated as a membrane when containing1 to 6 percent by weight polyethyleneglycol, the membrane having aporosity in the microfiltration range.

The polymer may be characterized as being hydrophobic in bulk form. Thebulk or powdered form of the polymer is non-wettable or "waterrepelling". When precipitated into a membraneous form in themicrofiltration range, these membranes are hydrophilic or spontaneouslywettable or hydrophilic or spontaneously wettable when they contain 1 to6 percent by weight of polyethyleneglycol.

This definition is not limiting of the characteristics of the membrane,as other reagents will accomplish the same function as thepolyethyleneglycol where needed. Rather, this limitation is useful incharacterizing the polymer from which the membrane is made. However,upon complete extraction of the polyethyleneglycol from the membrane,the membrane remains as a completely functioning membrane which issometimes no longer hydrophilic. On the other hand, a similar amount ofpolyethyleneglycol in membranes made from structurally similar polymers,for example polysulfone, does not produce a wettable membrane. Hence,the properties of the membrane made in accordance with the subjectinvention are completely unexpected.

A common measure of hydrophobicity of polymers is water absorption bythe bulk polymer within 24 hours or at equilibrium as set out in ASTMD570 (standard method to measure water absorption by polymers). Thereis, however, no commonly agreed definition of hydrophobic andhydrophilic polymers. We define a hydrophobic polymer as one absorbingless than 0.5% of its weight of water within 24 hours, and 4% or less atequilibrium. The surface of a solid piece of such a polymer willtypically not wet, and a water drop placed on such an inclined surfacewill roll off without tailing.

Literature data on the water absorption of a number of polymers is givenin the following table.

                  TABLE I                                                         ______________________________________                                        Water Absorption of Polymers                                                  (Source: Modern Plastics Encyclopedia 1985)                                   Polymer         24 Hour (%) Saturation (%)                                    ______________________________________                                        PTFE             0.01                                                         Polyacrylonitrile                                                                             0.3                                                           Polyetherimide   0.25        1.25                                             Thermoplastic polyimide                                                                        0.24                                                         Polyamide-imide  0.28       4.0                                               Polysulfone     0.3         0.7                                               Polyethersulfone                                                                              0.4         2.5                                               Aromatic polyamide                                                                            0.4         4.2                                               Nylon 66        1.1         8.5                                               Cellulose acetate                                                                             2.7                                                           ______________________________________                                    

By the above definition, polyetherimide, polysulfone, andpolyethersulfone are hydrophobic while nylon 66, aromatic polyamide andcellulose acetate are hydrophilic.

The polymers used for the membranes of the present invention arecharacterized by a specific set of properties. They are hydrophobicaccording to the above definition, but they form hydrophilic,spontaneously wettable microporous membranes or hydrophilic,spontaneously wettable microporous membranes when the membranes are madeso as to contain polyethyleneglycol or other polymer additives.

U.S. Pat. No. 4,413,074 to Brunswick Corp. describes a post treatmentfor preformed porous polysulfone membranes to render them waterwettable. It is emphasized in that patent that the treatment isespecially suitable to polysulfone. In fact, with polysulfone membranesthis treatment or addition of substantial amounts of surfactants isabsolutely necessary for wettability. With polyethersulfone, however, inthe context of the present invention, much milder and simpler means canbe used to make the membrane wettable. These could be, for example,inclusion of some polyethyleneglycol in the membrane or inclusion ofsmall amounts of polyvinylpyrrolidone in the membrane matrix.

We define microporous membranes as membranes having pore size ratings of0.02um to 20um. Such membranes will not reject salts from feed solutionsnor will they retain dissolved high molecular weight substances such asproteins. We define wettability as spontaneous absorption of water intoat least 50% of the exposed membrane area within not more than 10seconds when the membrane is placed onto the surface of stagnant water.

We have found that polymers fulfilling the requirements of the presentinvention will typically have 24 hour water absorption of about 0.2 to0.4% and equilibrium water absorptions of more than about 2% but lessthan about 4%. More specifically, in order to determine whether apolymer is suitable and falls within the definition of the presentinvention, the following test, useful in characterizing the membrane, isapplied (herinafter called the Test Method).

The Test Method

A hydrophobic polymer according to the above definition is dissolved ina polar aprotic solvent such as dimethylformamide, dimethylacetamide, orN-methyl pyrrolidone. Polyethyleneglycol 400 (PEG) is added and themixture is stirred to homogeneity. Concentrations are adjusted so thatthe polymer is about 10 to 13% of the mixture, solvent is about 17 to20% and PEG is about 65 to 70%. The mixture is cast at a thickness ofabout 10 mils onto a clean glass plate and subjected to about 50-80%relative humidity air at ambient temperature until the film becomesopaque. The membrane is then immersed into ambient water overnight towash out solvents, and dried in ambient air two hours and in a 70° Coven for one hour. Membranes made this way will have porosity in themicroporous range and will typically contain 2 to 6% residual PEG.Polymers according to the present invention are characterized by themicroporous membrane prepared as described above as being spontaneouslywettable.

The Test Method as described will reveal surprising differences amongstapparently similar polymers. After studying Table I it is not surprisingthat nylon 66 membranes are inherently wettable. However, it issurprising and unexpected that polymers with much lower waterabsorptions will form wettable membranes. Even more surprising, polymerswhich are very similar in chemical composition and water absorption willgive membranes with different wettability properties. Polymers accordingto this invention include, but are not limited to, aromaticpolyethersulfones, preformed polyimides, and polyamide-imides derivedfrom fully aromatic polyacids.

One preferred polyethersulfone polymer of the present invention ispolyethersulfone (sold under the tradename Victrex™). Its molecularstructure, ##STR1## is very similar to that of polysulfone (sold underthe tradename Udel™): ##STR2##

Table I shows that the two polymers have quite similar water absorptionproperties in bulk form and both are hydrophobic. Still, onlypolyethersulfone will provide wettable microporous membranes by theaforementioned Test Method, while polysulfone will give hydrophobicmembranes. Polysulfone and polyethersulfone have often been mentionedtogether or in similar contexts in the open and patent literature,especially in the context of ultrafiltration membranes.

It is implied in these references that membranes from both polymersbehave identically in the ultrafiltration range of porosity. Forexample, a number of methods have been devised to make both polysulfoneand polyethersulfone ultrafiltration membranes wettable. In this tighterrange of porosity, neither membrane is spontaneously wettable.References on point are U.S. Pat. No. 2,651,030 to Desauliners, issuedMar. 21, 1972, U.S. Pat. No. 3,632,404 to Desauliners, issued Jan. 4,1972, German patent No. 2,829,630 and Japanese patent No. 8,250,507. Arecent publication pointed out the better heat stability ofpolyethersulfone ultrafiltration membranes. (Kai et al AVCS Syrup Series281, p. 281, p. 21, 1985).

Similar to the relationship between polyethersulfone and polysulfone,polyether imide, polyamide-imide and thermoplastic polyimide havesimilar water absorption properties in bulk form, but only the lattertwo provide hydrophilic microfiltration membranes.

Apparently, subtle differences in the hydrophobicity of polymers arerevealed much more quickly and visibly when made into high surfacemorphologies such as microporous membranes. Such differences have notbeen noticed before and are the basis for the Test Method of the presentinvention.

Referring specifically to the aforementioned group of polymers,preferably, the aromatic polyether is a polyethersulfone having therecurring group ##STR3## This particular polyethersulfone may bepurchased under the trade name "Victrex"™. This polyethersulfone isgenerally resistant to aliphatic alcohols, some chlorinatedhydrocarbons, and some aromatic hydrocarbons.

Preferably, the polyimide includes the recurring group ##STR4## wherein10% to 90% of the R groups are ##STR5## and the remaining R groupsinclude either ##STR6##

Such a polyimide is sold by Upjohn Company under the trade name"Polyimide 2080"™. Polyimide 2080 is resistant to most organic solventsand dilute acids. However, polyimides are not recommended for long termexposure to caustic solutions, including ammonia and hydrazine.

Preferably, the polyamide-imide is a compound sold by Amoco Corporationunder the trade name "Torlon"™. The molecular structure of the polymerconsists essentially of the following repeating unit: ##STR7## where Ris largely aromatic. Polyamide-imide is available at a reasonable cost,and is soluble in commonly used solvents such as N,N dimethylformamide(DMF) or N-methylpyrrolidone. Generally, the polyamide-imide isunaffected by aliphatic and aromatic hydrocarbons, chlorinated andfluorinated hydrocarbons, and most acids and bases. It is attacked bysome high temperature caustic systems and high temperature acid systems.

Other polymer additives can be used in the Test Method in addition topolyethyleneglycol. Regarding polyethersulfone, we have found thatpolyethersulfone membranes can be made wettable by a variety of simplemeans. Thus, in addition to membranes made according to the Test Method,small amounts of polyvinylpyrrolidone (for example, less than 1% of thepolyethersulfone weight) can be incorporated into the membrane.Microporous membranes made this way will stay completely wettable evenafter exhaustive extraction with water or alcohols, or extensive heattreatments. Much higher concentrations of polyvinylpyrrolidone arenecessary to make polysulfone membranes wettable.

The instant invention further provides a method of making a microporousmembrane including the steps of dissolving a polymer in a suitablesolvent such as dimethylformamide to form a polymer solution, thepolymer being selected from the group of polymers which are inherentlyhydrophobic in bulk form and hydrophilic as a microporous membrane orhydrophilic as 1-6% PEG containing polymerized membranes having porosityin the microfiltration range. A thin film is formed of the polymersolution. The polymer is precipitated as a microporous membrane anddried.

A suitable plasticizer may be added to the polymer solution. Examples ofsuitable plasticizers are glycerine or polyethyleneglycol. For example,the ratio of the mixture of ingredients may be:

polymer 12%

PEG/glycerine 68%

solvent 20%

Polyethyleneglycol can also be used as a pore forming agent.

Preferably, when the polymer is polyamide-imide the solution includes11% to 15% by weight polyamide-imide, 40% to 45% by weight DMF and 44%to 48% by weight PEG.

Generally, the polymer solution is cast on a moving belt and subjectedto conditions of controlled air velocity, belt temperature, and relativehumidity. The liquid film of the polymer imbibes sufficient water toaffect initial precipitation of the polymer from the solvent. Finalprecipitation, which forms the microporous membrane, occurs in a quenchbath which contains a strong nonsolvent such as water. The formedmicroporous membrane may subsequently be dried in an oven.

Preferably, the solution for microfiltration membranes is cast on a flatsurface at a 10 to 12 mil thickness. The solution is allowed tocoagulate under a 60% to 70% relative humidity ambient air.

Once coagulation is completed, the solvent is removed by immersing themembrane in a nonsolvent to leach the solvents out. During this time,coagulation can also be completed. The nonsolvent may be water, as wellas other polar solvents in which the polymerized membrane will remainstable while the PEG and DMF will dissolve.

Once coagulation and leaching of the excess solvents are completed, themembrane is dried. Preferably, the membrane is dried at a temperatureranging from ambient to 70° C.

Pursuant to the method discussed above, the membranes may be made oneither a small or large scale. Large scale manufacturing can beaccomplished by casting the membrane on an endless belt and leaching thesolvent from the formed membrane in large size water baths.

As a further illustration, the polyamide-imide membrane made to includepores having a size rating in the microfiltration range of between about0.02 microns to about 20 microns is inherently wettable, havingexcellent water permeation rates and having exceptional thermalstability. This is surprising since the polymer per se is hydrophobic inthe bulk resin form. However, as shown in the examples, these formedmembranes show water flow rates comparable to or better than those ofthe best commercial microfiltration membranes. Thermal stability is farsuperior to that of available membranes, thus offering new potentialapplications for microfiltration membranes.

The microfiltration membranes made in accordance with the instantinvention are very useful for aqueous filtrations. If necessary, thepolyamide-imide membrane can also be made hydrophobic by posttreatments, such as by application of silcone compounds. Theposttreatment of the silicon compound is throughout the pore surfaces ofthe membrane thereby making the membrane hydrophobic on its interiorpore surfaces as well as on the outer surface of the membrane. Thesemicroporous membranes can be subjected to temperatures of 250° C. andhigher for extended periods of time without major changes in filtrationperformance.

The polyamide-imide microporous membranes show excellent water flowversus bubble point characteristics. The mechanical and hydrolyticproperties of these membranes can be substantially improved by heatcuring of the membrane. By curing the membrane at 260° C. for 4 to 20hours, tensile strength increases at least 50%. In addition, thehydrolytic stability improves to the extent that the membrane canwithstand repeated autoclavings and steamings. Alternatively, the curingcan be done on the resin before it is made into a membrane with similarresults. Heat curing has little effect on flow properties of themembranes.

Membranes can be manufactured from either cured or uncured polymer.Either type of membrane can be made into devices such as capsules andcartridges by pleating and sealing of the membranes.

The following examples illustrate the invention in greater detail. Sincethe following examples are for the purpose of illustrating theinvention, they shall not be construed as limiting the invention in anyway.

EXAMPLES Definitions

    ______________________________________                                        Water bubble                                                                           The water bubble point is a test to measure the                      point:   largest pore size of a filter, based on the air pres-                         sure necessary to force liquid from the pores of a                            wetted filter. The larger the pore, the less pressure                         to vacate it. Air passing through the empty pore is                           detected as bubbles. The differential pressure to                             force the first bubble out is defined as the                         bubble point. The relationship be-                                                     tween the bubble point pressure and the diameter                              of the large pores is given by:                                                ##STR8##                                                                     air surface tension, θ is the liquid solid contact                      angle and D is pore diameter.                                        Air flow:                                                                              Air flow depends chiefly on the differential pres-                            sure, and on the total porosity and area of a filter.                         The total amount of air that can be filtered is also                          a function of contamination in the flow. The                                  Gurley and Frazier tests are two common                                       measurements of filter air flow.                                     Water flow:                                                                            The water flow/flux test measures the rate at                                 which water will flow through a filter - a variable                           of differential pressure, porosity, and filter area.                          Flow rates are commonly expressed in either gal-                              lons/minutes/feet squared or milliliters/min-                                 utes/centimeters squared at a given pressure.                        ______________________________________                                    

EXAMPLE 1 Polyethersulfone Membrane 0.2u

polyethersulfone (Victrex™ 5200) dimethylformamide andpolyethyleneglycol 400 were mixed in the ratio 13:18:69. The mixture wasstirred to homogeneity and cast at 10-12 mil on glass or stainlesssteel. It was subjected to 60-70% relative humidity ambient air until itbecame opaque. The film was then immersed in water to completecoagulation and leach out excess solvent, for 2-12 hours. It was thendried at ambient to 70° C.

The membrane obtained was spontaneously water wettable. It exhibited100% bacteria retention when challenged with 10⁷ /cm² of Pseudomonasdimunitae. The membrane had the following flow characteristics:

    ______________________________________                                        Kerosene Bubble Point                                                                           22 psi                                                      Water Bubble Point                                                                              53 psi                                                      Air Flow          2.7 lit/cm.sup.2 -min at 10 psi                             Water Flow        23 ml/cm.sup.2 -min at 10 psi                               ______________________________________                                    

Nuclear magnetic resonance of the dissolved membrane showed that itcontained 5% by weight of polyethyleneglycol.

COMPARATIVE EXAMPLE 1. Polysulfone Membrane 0.2u

A polysulfone (Udel™ 3500) membrane was made by a procedure essentiallythe same as in Example 1.

Nuclear magnetic resonance of the dissolved membrane showed that itcontained 6% polyethyleneglycol. However it was totally hydrophobic.

Membrane performance was:

    ______________________________________                                        Kerosene Bubble Point                                                                            34 psi                                                     Water Flow         17.8 ml/cm.sup.2 -min at 10 psi                            (After prewetting with ethanol)                                               ______________________________________                                    

EXAMPLE 2 Polyethersulfone Membrane 0.45u

A casting solution was prepared by mixing 11.5% polyethersulfone(Victrex™ 5200) with 25% N-methyl-pyrrolidone, 68% polyethyleneglycol,and 0.5% glycerine. The membrane was cast and set as in Example 1. Themembrane obtained was spontaneously water wettable.

Flow characteristics were:

    ______________________________________                                        Kerosene Bubble Point                                                                           17 psi                                                      Water Bubble Point                                                                              36 psi                                                      Air Flow          4.8 lit/cm.sup.2 -min at 10 psi                             Water Flow        42 ml/cm.sup.2 -min at 10 psi                               ______________________________________                                    

EXAMPLE 3 Polyethersulfone Membrane 0.1u

A casting solution was prepared by mixing 15% polyethersulfone (Victrex™5200) with 18% dimethylformamide, 66.5% polyethyleneglycol and 0.5%glycerine. The membrane was cast and formed as outlined in Example 1.The membrane obtained was spontaneously water wettable. Performance wasas follows:

    ______________________________________                                        Kerosene Bubble Point                                                                           37.5 psi                                                    Water Bubble Point                                                                              88.8 psi                                                    Air Flow          1.7 lit/cm.sup.2 -min at 10 psi                             Water Flow        9 ml/cm.sup.2 -min at 10 psi                                ______________________________________                                    

EXAMPLE 4 Polyamide-Imide Membrane 0.2u

Polyamide-imide (Torlon™ 4000TF), N,N-dimethylformamide andpolyethyleneglycol were mixed in the ratio of 11.5:40:48.5. The mixturewas stirred to homogeneity and cast at a 10-12 mil thickness on glass ora stainless steel belt as in Example 1. The cast solution was subjectedto 60-70% relative humidity ambient air until phase separation occurred.The membrane was then immersed in water to complete coagulation andleach out excess solvents. The membrane was then dried at ambient to 70°C.

The membrane was spontaneously wetted when placed onto the surface ofstagnant water. It showed the following performance characteristics.

    ______________________________________                                        Kerosene bubble point                                                                           23 psi                                                      Water bubble point                                                                              54 psi                                                      Air flow          2.3 lit/cm.sup.2 -min at 10 psi                             Water flow        19 ml/cm.sup.2 -min at 10 psi                               ______________________________________                                    

EXAMPLE 5. Polyamide-Imide Membrane 0.45u

A casting mixture was prepared by mixing 11.5% polyamide-imide (Torlon™4000TF) with 44% dimethylformamide and 44.5% polyethyleneglycol 400. Amembrane was cast and set essentially as described in Example 4. Themembrane obtained was inherently wettable and showed the followingcharacteristics:

    ______________________________________                                        Kerosene bubble point                                                                           13 psi                                                      Water bubble point                                                                              30 psi                                                      Air flow          4.5 lit/cm.sup.2 -min at 10 psi                             Water flow        36 ml/cm.sup.2 -min at 10 psi                               ______________________________________                                    

EXAMPLE 6. Polyamide-Imide Membrane 3u

A casting mixture was prepared consisting of 8% polyamide-imide (Torlon™4000TF), 45% dimethylformamide and 47% polyethyleneglycol 400. Amembrane was cast and formed as set essentially out in Example 4. Themembrane obtained was inherently hydrophilic and exhibited the followingcharacteristics:

    ______________________________________                                        Kerosene bubble point                                                                           2.4 psi                                                     Water bubble point                                                                              6.2 psi                                                     Air flow          219 lit/cm.sup.2 -min at 10 psi                             Water flow        144.6 ml/cm.sup.2 -min at 10 psi                            ______________________________________                                    

COMPARATIVE EXAMPLE 6. Polyether-Imide Membrane

A casting mix was prepared as in Example 6 except polyether-imide(Ultem™ 1000) was used instead of polyamide-imide. Kerosene bubble pointwas 3.5 psi and the membrane was totally hydrophobic.

EXAMPLE 7. Thermoplastic Polyimide Membrane

A mixture was prepared consisting of 13% thermoplastic polyimide (Upjohn2080D), 40% dimethylformamide, ethyleneglycol, 8% tetrahydrofuran and27% N-methylpyrrolidone (by weight). The solution was cast and set up asin Example 1. The membrane obtained was wettable and showed thefollowing performance:

    ______________________________________                                        Kerosene Bubble Point                                                                           20.8 psi                                                    Air Flow          2.0 lit/cm.sup.2 -min at 10 psi                             Water Flow        9.9 ml/cm.sup.2 -min at 10 psi                              ______________________________________                                    

EXAMPLE 8. Thermoplastic Polyimide Membrane

A solution was prepared containing 13% thermoplastic polyimide (Upjohn2080D), 46% dimethylformamide, 6% ethyleneglycol, 2% tetrahydrofuran and33% N-methylpyrrolidone (by weight).

A membrane was prepared as in Example 1. The membrane obtained was waterwettable and showed the following performance characteristics:

    ______________________________________                                        Kerosene Bubble Point                                                                           14.6 psi                                                    Air Flow          3.8 lit/cm.sup.2 -min at 10 psi                             Water Flow        23.7 ml/cm.sup.2 -min at 10 psi                             ______________________________________                                    

EXAMPLE 9. Heat Stability of Polyamide-Imide Membrane

Membrane samples made in accordance with the instant invention weresubjected to heat treatment in an oven at different temperatures and fordifferent periods of time. Performance was measured before and aftertreatment with the following results.

    ______________________________________                                                Performance                                                                     Water    Air Flow  Water Flow                                       Treatment BP psi   1 pm/5 psi                                                                              ml/min-cm.sup.2                                                                        Wetting                                 ______________________________________                                        None      9.8      40        400      Instant                                 210° C./16 hr                                                                    10.6     43        400      Instant                                 240° C./5 hr                                                                     9.9      41        385      Instant                                 280° C./3 hr                                                                     9.0      46        360      Instant                                 ______________________________________                                    

EXAMPLE 10 Effect of Heat Curing on Polyamide-Imide Membrane Properties

0.45um membranes were heat cured at 269° C. for 20 hours. They were thenrepeatedly autoclaved at 118° C. for 20 minutes and typical performanceand mechanical characteristics measured. Results summarized in thefollowing table show that the membranes are little affected by theautoclaving.

    ______________________________________                                                Tensile   Elongation                                                                              Bubble  Water                                             Strength  at Break  Point   Flow                                      Treatment                                                                             (psi)     (%)       (H.sub.2 O, psi)                                                                      (ml/cm.sup.2 /min)                        ______________________________________                                        None     730      8.4       23      77                                        Cured   1700      6.8       24      84                                        Autoclave                                                                             1300      10.2      23      80                                        (1 cycle)                                                                     Autoclave                                                                             --        --        23      80                                        (2 cycles)                                                                    Autoclave                                                                             --        --        22      78                                        (3 cycles)                                                                    Autoclave                                                                             1900      8.2       22      75                                        (4 cycles)                                                                    ______________________________________                                    

EXAMPLE 11 Blend Polyethersulfone Membrane

A membrane was prepared as in Example 1, except thatpolyvinylpyrrolidone was substituted for 0.7% of the polyethersulfoneweight. The membrane obtained was inherently wettable and, unlike themembrane of Example 1, retained its wettability upon exhaustiveextraction with isopropanol or extended exposure to 170° C. in an oven.Polysulfone membranes needed substantially higher concentrations ofpolyvinylpyrrolidone to induce similar wettability.

EXAMPLE 13. Post Treated Polyethersulfone Membrane

A membrane was prepared as in Example 1. The membrane was then posttreated with a dilute aqueous solution of polyvinylalcohol and wassubsequently crosslinked. This membrane retained wettability afterprolonged extraction in isopropanol and extended heat treatment as inExample 11.

The results indicate that the invention provides increased flow andthroughput. It has also been found that the instant invention providesgood temperature properties and handling, resulting in increasedpracticality for use.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology which has been used is intended to bein the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A microporous membrane comprising a polymer whichin bulk form is hydrophobic and has an equilibrium water absorptionranging from about 2% to about 4% and which is blended with a polymeradditive in an amount effective to make the membrane when formed anddried inherently water wettable with a pore size range such that themembrane does not retain or reject dissolved proteins or salts fromaqueous feed solutions, the thus dried membrane being post treated withaqueous polyvinyl alcohol and subsequently crosslinked such that thepost treated membrane retains wettability after prolonged extraction inisopropanol or extended heat treatment.
 2. The membrane of claim 1wherein said polymer additive is selected from polyethleneglycol andpolyvinylpyrrolidone.
 3. The membrane of claim 1 wherein saidhydrophobic polymer is selected from polyethersulfones and polyimides.4. The membrane of claim 1 wherein said hydrophobic polymer is apolyamide-imide.
 5. A membrane according to claim 1 comprising apolyethersulfone of the general formula ##STR9##
 6. A membrane accordingto claim 1 comprising a polyimide of the general formula ##STR10## where10% to 90% of R is ##STR11## and the remaining R groups are ##STR12## 7.A membrane according to claim 1 comprising a polyamide-imide of thegeneral formula ##STR13## where R is aromatic.
 8. A method of producinga microporous, inherently water wettable dry membrane, comprising:(a)dissolving in a polar aprotic solvent a polymer which in bulk form ishydrophobic and has an equilibrium water absorption of about 2% to about4%; (b) adding a pore forming agent to the solution; (c) blending withthe solution a polymer additive in an amount effective to make themembrane when formed and dried inherently water wettable with a poresize range such that the membrane does not retain or reject dissolvedproteins or salts from aqueous feed solutions; (d) casting the solutionin a thin layer; (e) humidifying the solution sufficiently toprecipitate a membrane; (f) drying the membrane; and (g) treating thedried membrane with aqueous polyvinyl alcohol and crosslinking thetreated membrane such that the crosslinked membrane retains wettabilityafter prolonged extraction in isopropanol or extended heat treatment. 9.The method of claim 8 wherein the polymer additive dissolved into thesolution is selected from polyvinylpyrrolidone and polyethyleneglycol.10. The method of claim 8 wherein the polymer dissolved in the solventis selected from polyethersulfones and polyimides.
 11. The method ofclaim 10 wherein the polymer dissolved in the solvent is selected from apolyethersulfone of the general formula ##STR14## and a polyimide of thegeneral formula ##STR15## where 10% to 90% of R ##STR16## and theremaining R groups are ##STR17##
 12. The method of claim 8 wherein thepolymer dissolved in the solvent is a polyamide-imide.
 13. The method ofclaim 8 wherein the polymer dissolved in the solvent is selected from apolyamide-imide of the general formula ##STR18## where R is aromatic.